The present invention relates generally to diffraction-free beams of coherent light. More specifically, the present invention relates to the provision of a Mathieu-Gaussian beam, which can be used in optical scanners or optical communication systems.
Optical scanning technologies have enjoyed widespread use in recent years. Because of this widespread use, substantial efforts have been directed to finding improved techniques for optical scanning. Additionally, the field of optical communications has also grown in recent years, and likewise has spurred a desire to find new optical technologies to improve such communication.
For example, in the field of barcode scanning, many different types of scanning beams have been experimented with to obtain ideal scanning characteristics of the beams. Traditionally, a beam having a Gaussian profile is readily obtained from a laser diode and has advantages in scanning applications. Also, beams generated by way of edge-emitting laser diodes generally have elliptical cylindrical symmetry, also known as confocal symmetry, which is easily describe in terms of confocal coordinates, and which is advantageous in scanning elongated objects such as bars within a barcode, such as a universal product code (UPC) symbol, elements of a portable data file (PDF 417), or the like.
Generally, a beam having confocal symmetry and Gaussian properties is oriented in a manner such that the beam""s major axis is aligned with the axis of elongation of each element of a barcode, where the axis of elongation is the axis parallel to the longest side of each bar within a barcode. Such scanning is accomplished as a beam, incident upon a symbol (e.g., a barcode), which lies in a plane parallel to the direction of propagation of the beam. The dimension of the beam cross-section at the plane containing the symbol is generally referred to as the xe2x80x9cspot size.xe2x80x9d Because of the relative similarity of an elliptical beam""s spot size to the size and shape of the bars of a barcode, it is able to produce a reflected signal more readily read by conventional barcode readers.
One of the requirements for effective scanning is an adequate working range, in other words the range of distances from the scanner that the barcode symbol may be placed and still be decoded. It is typically only over a limited working range that the cross-section of the beam has an appropriate size and shape to allow reliable code reading. This is particularly evident where the beam is provided by way of a laser diode, since these exhibit substantial astigmatism. Typically, the application of shaping optics applied to such a beam creates a scanning beam in which the cross-section varies with distance from the scanner. The point at which the x-dimension of the beam cross-section is narrowest (the xe2x80x9cx-waistxe2x80x9d) may be at a different distance from that at which the y dimension is narrowest (the xe2x80x9cy-waistxe2x80x9d). Such astigmatism can be problematic for scanning over substantial distances.
It is well known that Gaussian beams, when used for scanning a target, provide a signal with excellent contrast. However, Gaussian beams suffer from limited working ranges, and the smaller the beam waists are designed to be, the shorter the working ranges become. Since the working ranges are effectively those where the beam cross-sections remain essentially constant, these correspond generally with the Rayleigh ranges.
In one case, it has been found advantageous to use a combination of Bessel and Gaussian beams to produce a Bessel-Gaussian beam. One such technique is described in copending application Ser. No. 09/867,399, filed May 31, 2001 by Bergstein et al. entitled xe2x80x9cBeam Shaping for Optical Scannersxe2x80x9d, the disclosure of which is hereby incorporated by reference in its entirety. In this copending application, the technique whereby a Bessel-Gaussian beam, which is a coherent combination of a Bessel beam and a Gaussian beam, is described. This Bessel-Gaussian beam enjoys the advantages of Gaussian and non-Gaussian beams alike. One major advantage of the Bessel-Gaussian beam is the fact that it is essentially diffraction-free.
However, in some applications wherein a diffraction free beam is desired, the ringed structure of the Bessel beam that is used to form the Bessel-Gaussian beam for scanning purposes may not be desirable. In such cases, invariant optical fields (IOFs) may be desirable because under ideal conditions, they are essentially diffraction free, propagating essentially indefinitely without a change in their transverse intensity distribution. One such IOF known as a Mathieu beam is discussed in J. C. Gutierrez-Vega, M. D. Iturbe-Castillo, and S. Chxc3xa1vez-Cerda, xe2x80x9cAlternative Formulation for Invariant Optical Fields: Mathieu Beams,xe2x80x9d Optics Letters, Vol. 25, No. 20, 1493-95, (Oct. 15, 2000). The Mathieu Beam is one of many solutions to the Helmholtz wave equation in elliptical cylindrical coordinates. The Mathieu Beam has infinite transverse extension in one direction, when the ideal model is considered. Thus, the ideal Mathieu beam is unsuited for practical application such as barcode reading.
It is an object of the present invention to provide a scanning beam, and source thereof which is usable with a wide range of optical code densities regardless of where the target symbol is located in the working range of the code reader.
It is another object of the present invention to provide a laser beam which is essentially diffraction free.
It is another object of the present invention to provide a laser beam which has an invarient optical field through the working range of a code reading or communications system in which it is employed.
It is another object of the present invention to provide a laser beam having a zone of illumination which is of essentially constant size through the working range of a code reading or communication system in which it is employed.
It is another object of the present invention to provide a laser beam for scanning barcodes which has an essentially constant, elliptically-shaped zone of illumination throughout the working range of the code reader.
These and other objects and features will be apparent from the following written description and drawings. It being understood that specific embodiments of the present invention may achieve only one or some of the objects stated in this application, and may achieve additional objects not stated in this application.
It would be desirable to produce a beam that has the advantages of the inherent ellipticity and diffraction-free nature of the ideal Mathieu beam, while controlling the infinite transverse extension in one direction of the beam. Such a beam should also maintain an essentially constant irradiance (W/m2), which is radiant power (generally described in terms of Watts) per area (generally described in square meters).
Such a beam would also be desirable in the field of optical communications, as the properties that make such a beam desirable for scanning applications also make it desirable for communications purposes. Specifically, a beam that is essentially diffraction free, having a nearly constant irradiance and spot size of a large distance would be usefully in many point-to-point communications systems.
Accordingly, the present invention achieves the foregoing objectives by providing a beam, and a method for making the beam that utilizes the advantages of the Mathieu beam, while controlling its infinite transverse extension in one direction. The beam created in accordance with the present invention is essentially diffraction free, and has a nearly constant irradiance and spot size over large distances.
The present invention makes use of Gaussian apodization to produce a Mathieu-Gaussian beam which has a spot size and irradiance that remains substantially constant over a varying distance. The Mathieu-Gaussian beam has a long elliptical irradiance profile, which may be used in barcode scanning and communications applications. In accordance with an embodiment of the present invention, the Mathieu-Gaussian beam may be used to generate a single scan line in the far field for scanning barcodes. Also in accordance with an embodiment of the present invention, the Mathieu-Gaussian beam may also be used in the near field to produce a desirable narrow raster scan pattern or other omni- or multi-directional pattern.
In accordance with one embodiment of the present invention, an apparatus for generating a scanning beam is provided. The apparatus includes a source of an apodized Mathieu beam of coherent light and a means for scanning the beam across a target optical code. The coherent apodized Mathieu beam may be created by a laser source having a Gaussian profile impinging upon a Mathieu filter in the optical path of the laser beam.
In accordance with another embodiment of the present invention, an apparatus for generating a laser beam is provided. The apparatus comprises a means for generating a beam of laser light, a means for introducing Gaussian apodization to the beam, and a means for filtering the beam to produce confocal symmetry in the beam. The means for filtering may comprise a Mathieu filter.
In accordance with yet another embodiment, the present invention provides a method for creating a Mathieu-Gaussian beam. The method comprises the steps of generating a beam, filtering the beam, and apodizing the beam. These steps may be performed in a single system either simultaneously, or in any order, to create a Mathieu-Gaussian beam. The method may include diffracting the beam to create a diffracted beam having confocal symmetry.
In accordance with a further embodiment of the present invention, a Mathieu-Gaussian laser beam is provided that has an illumination pattern with a central spot and at least one pair of side lobes. The laser beam may be approximately diffraction free and while propagating through a working distance and/or approximately constant throughout the working distance.
In accordance with another embodiment of the present invention, a laser beam for scanning a target barcode is provided. The laser beam is an apodized Mathieu beam having a central spot and at least one pair of side lobes which define a zone of illumination. The zone is elongated in a direction that is generally perpendicular to a direction in which the target barcode is scanned.
In accordance with yet another embodiment of the present invention, a method for designing a scanning laser beam source for optical code reader. In the method, a laser diode, optical system, and aperture are selected for producing a Gaussian-Mathieu beam having a central spot and at least one pair of side lobes defining a zone of illumination. The zone of illumination is relatively constant through the working range of the optical code reader, and is elongated having an aspect ration selected in accordance with the anticipated dimensions of target codes to be read by the code reader. The beam may be essentially diffraction free through the working range of the optical code reader.